size question 245/40R18 vs 255/35r18 ??? what is the difference ???

plug the sizes in:

http://www.1010tires.com/TireSizeCalculator.asp

or http://www.miata.net/garage/tirecalc.html

245/40 is more than 1/2 inch larger in diameter
255/35 will cause 2.7% difference in speed

Basically, the 255/35 is smaller than stock.. 245/40 is closer to stock.

As a reference point, matching staggered wheels are 245/40 and 265/35

plug the sizes in:

http://www.1010tires.com/TireSizeCalculator.asp
Thankyou , awsome find